shmop

, shmat

, shmdt

Synopsis

Description

The shmat() function attaches the shared memory segment associated with the shared
memory identifier specified by shmid to the data segment of the calling
process.

The permission required for a shared memory control operation is given as
{token}, where token is the type of permission needed. The types of
permission are interpreted as follows:

00400 READ by user
00200 WRITE by user
00040 READ by group
00020 WRITE by group
00004 READ by others
00002 WRITE by others

See the Shared Memory Operation Permissions section of Intro(2) for more information.

For shared memory segments created with the SHM_SHARE_MMU or SHM_PAGEABLE flags, the
default protections cannot be changed so as to prevent a single process
from affecting other processes sharing the same shared segment.

When (shmflg&SHM_SHARE_MMU) is true, virtual memory resources in addition to shared memory
itself are shared among processes that use the same shared memory.

When (shmflg&SHM_PAGEABLE) is true, virtual memory resources are shared and the dynamic
shared memory (DISM) framework is created. The dynamic shared memory can be
resized dynamically within the specified size in shmget(2). The DISM shared memory
is pageable unless it is locked.

The shared memory segment is attached to the data segment of the
calling process at the address specified based on one of the
following criteria:

If shmaddr is equal to (void *) 0, the segment is attached to the first available address as selected by the system.

If shmaddr is equal to (void*)0 and ( shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is true, then the segment is attached to the first available suitably aligned address. When (shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is set, however, the permission given by shmget() determines whether the segment is attached for reading or reading and writing.

If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is true, the segment is attached to the address given by (shmaddr- (shmaddr modulus SHMLBA)).

If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is false, the segment is attached to the address given by shmaddr.

The segment is attached for reading if (shmflg&SHM_RDONLY) is true {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

The shmdt() function detaches from the calling process's data segment the shared
memory segment located at the address specified by shmaddr.

Shared memory segments must be explicitly removed after the last reference to
them has been removed.

Return Values

Upon successful completion, shmat() returns the data segment start address of the
attached shared memory segment; shmdt() returns 0. Otherwise, -1 is returned, the
shared memory segment is not attached, and errno is set to indicate
the error.